As disclosed in Patent Document 1, a drive apparatus for driving a driven unit such as a variable compression ratio mechanism, with a driving unit such as a motor, employs a wave motion gearing or strain wave gearing type speed reducer providing a greater reduction ratio, in order to reduce the size and output power of the driving unit and to improve the controllability.
This speed reducer or reduction gear includes a rigid gear of rigid body having internal teeth formed in an inside circumference, a wave generator disposed coaxially inside the rigid gear, and a flexible gear which is disposed coaxially between the wave generator and the rigid gear, which is deformed elastically by the wave generator into an elliptical shape, and which has external teeth formed in an outside circumference of the flexible gear and arranged to engage with the above-mentioned internal teeth at two positions in the direction of the major axis of the elliptical shape. In this structure, the rigid gear and flexible gear are arranged to rotate relative to each other by a rotational or angular amount corresponding to a teeth number difference between the internal teeth and external teeth, with respect to one revolution of the wave generator.